<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="../js/vue.js"></script>
</head>

<body>
    <div id="root">
        <p>FullName: {{fullName}}</p>
        <p>FirstName: <input type="text" v-model="firstName"></p>
    </div>
</body>
<script>
    new Vue({
        el: '#root',
        data: {
            firstName: 'Dawei',
            lastName: 'Lou',
            fullName: ''
        },
        watch: {
            firstName: {
                handler(newName, oldName) {
                    this.fullName = newName + ' ' + this.lastName;
                }
            },
            immediate: true
        }
    })





</script>

</html>